CryoEM of OPA1 helical Structures give perspective to mitochondrial dysfunction

Authors

• SB Nyenhuis
• X Wu
• MP Strub
• YI Yim
• AE Stanton
• B Canagarajah
• JA Hammer
• JE Hinshaw

Abstract

Dynamin Superfamily Proteins (DSPs) are mechanochemical GTPases which regulate many membrane fission, fusion, and trafficking events in the cell, such as endocytosis and mitochondrial network dynamics. Dynamin, the founding member, drives endocytosis, synaptic membrane recycling, and membrane trafficking events. It is thought to assemble as a helical polymer around the necks of budding vesicles and upon GTP hydrolysis, undergo constriction ultimately leading to membrane fission. Optic Atrophy 1, OPA1, among other roles, drives inner mitochondrial membrane fusion. OPA1 exists in two forms in vivo: the long-form which is tethered to the inner mitochondrial membrane (IMM) through a transmembrane domain, and the short-form that results from proteolytic cleavage in the inner membrane space. A balance of these forms drives IMM fusion. The mechanics of this process, remain unknown due to a lack of OPA1 structural information.
Using cryo-electron microscopy, we solved helical structures of the short form of OPA1 isoform-1 (s-OPA1) with and without nucleotide, revealing both the domain organization of s-OPA1 and its mode of higher order assembly. These helical assemblies exhibit nucleotide-dependent dimerization of the GTPase domains, a hallmark of DSPs. In contrast to other DSPs, OPA1 contains several unique secondary structures in the paddle domain that enhance its membrane association through monotopic, membrane-inserting helices. Novel structural features of OPA1 shed light on the effects of pathogenic point mutations associated with Dominant Optic Atrophy. Further, mutations chosen to disrupt OPA1 assembly interfaces and membrane binding cause mitochondrial network fragmentation in cell-based assays, demonstrating the biological relevance of these interactions.

Scientific Focus Area: Structural Biology

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